Jean-Claude Ruel

Factors influencing windthrow in balsam fir forests: from landscape studies to individual tree studies.

Abstract Windthrow hazard depends upon the interaction of numerous factors whose relative importance has yet to be assessed in the specific context of Eastern Canadian boreal forests. This paper presents results from three studies looking at their relative importance at different scales in the context of balsam fir stands. First, factors involved in a catastrophic windthrow are examined in order to assess their importance at a regional and local scale. Wind speeds were estimated from a numerical model and overlayed with windthrow, soil and stand maps. The two regions that were damaged presented many predisposing factors: high occurrence of shallow soils, of vulnerable species and of overmature stands. Moreover, both regions have been identified as high wind areas. No consistent effect of wind speed on windthrow could be demonstrated, but the increasing vulnerability of stands with increasing proportion of balsam fir in one region and increasing age in balsam fir stands in both regions was significant. In one region, shallow tills experienced more damage in comparison with deep tills but not in the other. The second study looks at windthrow in riparian buffer strips at a local level. It includes a wind tunnel study and a field monitoring in stands of similar species composition and age. Results have shown that windthrow after 7 years was very variable and did not differ with strip width or thinning in the strip. In fact, it was more closely related to locations exposed to stronger winds stressing the importance of wind exposure estimation. It also confirms the greater vulnerability of balsam fir. The last study addresses the identification of external indicators of mechanical weaknesses on individual trees. It involves dissections and winching of trees with and without defects. Results tend to indicate a reduction in resistance when cracks are present on balsam fir. These three studies provide some information to begin considering windthrow hazard in the management of balsam fir forests.

Windthrow in riparian buffer strips: effect of wind exposure, thinning and strip width

This paper discusses the effects of topography, riparian buffer strip width and thinning on the amount of windthrow over 9 years in balsam fir (Abies balsamea) stands. Monitoring of windthrow was conducted in 25 riparian areas representing five treatments: uncut control, thinned 20 m strip, unthinned 20, 40 and 60 m strips. Wind tunnel measurements were made on a topographical model of the study area to provide an estimate of local wind behaviour. A cluster analysis performed on the wind speed data led to the identification of four major topographic units of similar wind behaviour. Wind speed in valleys varied greatly depending upon the direction of the approaching winds and the presence of small topographic features. When the wind blows perpendicularly to the valley, wind speed tends to be lower but more variable than when it blows parallel to the valley. Windthrow was not related to strip width or thinning. The field study showed that windthrow 5 years after cutting was found to be correlated with the speed of winds blowing roughly perpendicular to the strips. Windthrow after 7 and 9 years, following an unusual wind event that occurred between years 5 and 7, was no longer correlated with this wind direction. High levels of damage were observed where the valley widened, offering less shelter to the buffer strips.

Establishment, growth and survival of natural regeneration after clearcutting and drainage on forested wetlands.

Natural regeneration may be disrupted by the rise of the water table in surface layers after clearcutting forested wetlands. A study was initiated on eight forested wetlands that were successively clearcut and drained 3 years later. The objectives were (1) to assess conifer and deciduous regeneration on waterlogged clearcut sites, (2) to determine the effect of water table level changes after clearcutting and drainage on the growth rate of advance regeneration, and (3) to determine if sphagnum moss growth rate was influenced by clearcutting, drainage and microrelief. 3 years after strip clearcutting, the area based inventory showed that softwood regeneration was abundant in the clearcut, but 84% of the seedlings were smaller than 30 cm and vulnerable to suppression from the competition. Indeed, the opening of the forest cover promoted massive invasion of pioneer species such as trembling aspen (Populus tremuloides Michx.) and white birch (Betula papyrifera Marsh.). Black spruce (Picea mariana (Mill.) B.S.P.) and red spruce (Picea rubens Sarg.) had difficulties maintaining their presence after clearcutting. The ratio hardwood stems : softwood stems changed from 1.1 in the forest to 3.8 in the clearcut. Balsam fir (Abies balsamea (L.) Mill.), larch (Larix laricina (Du Roi) K. Koch) and cedar (Thuja occidentalis L.) represented 93% of the softwood regeneration. 3 years after clearcutting, balsam fir advance regeneration had a significantly lower growth rate in the middle of the clearcut than near the edges of the clearcut. 2 years after drainage, seedling growth at 10 m from the drainage ditch was significantly greater than at 70 m from ditch. No significant differences were found between mineral and organic soil types, but height growth was positively correlated to C : N ratios of the individual site types. Abundant competition on richer sites limits the development of softwood regeneration. On a short term basis, competition more than watering-up seems a threat to softwood regeneration. Sphagnum growth rates measured with the crank-wire method showed no effect of clearcutting or drainage, but a higher growth rate in hollows (3.2 cm per year) compared to hummocks (2.0 cm per year). These growth rates indicate that sphagnum should not affect regeneration but could overgrow yearlings of slow growing species. # 2000 Elsevier Science B.V. All rights reserved.

Short term response of small mammals and forest birds to silvicultural practices differing in tree retention in irregular boreal forests.

Abstract: The effects of logging on wildlife have been evaluated mainly in landscapes dominated by even-aged forest stands, leaving the impacts of harvesting on wildlife in irregularly structured stands less documented. We assessed the response of small mammals and birds to 4 silvicultural treatments with different levels of tree retention (from < 10% to 73%) in old-growth boreal forests of eastern Canada, where approximately 70% of stands have irregular structure. The experimental design controlled for local variation in the abundance of forest birds and small mammals by pairing each experimental harvested stand with an uncut stand. We found that species richness of small mammals and birds did not vary according to tree retention. Moss cover and vertical cover influenced small mammal community composition, whereas forest bird assemblages responded to conifer basal area, vertical cover, and snag availability. Species associated with silvicultural practices entailing retention levels > 55% included those that prefer closed canopy, such as the brown creeper (Certhia americana), ruby-crowned kinglet (Regulus calendula), and red-backed vole (Myodes gapperi). Our study demonstrates that silvicultural practices with retention levels > 55% are the most suitable for mitigating the impact of logging on animal communities in old-growth irregularly structured stands.

Wood degradation after windthrow in a northern environment.

Severe windthrows often require salvage operations that can lead to increased costs. Given these extra costs, it is of paramount importance to make sure that wood degradation does not become so advanced that significant value loss is incurred. The rate at which wood deteriorates is a function of many factors, including species and climate. The study was conducted in a northern area affected by two partial windthrows. Logs from the damaged area were collected for two species, balsam fir (Abies balsamea) and black spruce (Picea mariana). Logs were classified into one of three degradation classes based on visual assessments. A sample of logs from standing trees was also collected. In total, 167 logs were sampled. Each log was sawn and one piece of lumber was selected from each to determine the bending strength and stiffness and the visual grade. The time since tree death, as determined from dendrochronology, ranged from 1 to 31 years. The visual grade of the lumber was not affected after 1 year but severe downgrades were observed after 4 years. Moisture content decreased rapidly for both species during the first year and continued to decrease until 4 years after mortality. No clear decrease in bending stiffness was identified even though such a tendency was noticed for older black spruce windthrows. Bending strength became variable after 4 years for balsam fir and was reduced after 4 years for black spruce. Windthrows older than 7 years will produce low visual grade timber of reduced bending strength and possibly of lower bending stiffness.

Probabilités de pertes des tiges individuelles, cinq ans après des coupes avec protection des petites tiges marchandes, dans des forêts résineuses du Québec

La coupe avec protection des petites tiges marchandes est un type de coupe partielle qui consiste generalement a recolter toutes les tiges d’un diametre a hauteur de poitrine (dhp) superieur a 15,0 cm, tout en conservant les tiges de plus petites dimensions. Le succes du traitement, applique a des forets resineuses mures dominees par le sapin baumier (Abies balsamea (L.) Mill.) ou l’epinette noire (Picea mariana (Mill.) Britton, Sterns & Poggenb.), repose en partie sur la capacite des tiges protegees a survivre. Un modele logistique mixte a ete calibre a partir de 27 blocs experimentaux etablis au Quebec. Ce modele identifie les variables qui conditionnent les probabilites de pertes des tiges individuelles protegees de 5,1 cm et plus de dhp, par mortalite sur pied ou par chablis, 5 ans apres des coupes avec protection des petites tiges marchandes. Les resultats indiquent que les probabilites de pertes apres traitement sont largement tributaires des caracteristiques du peuplement avant coupe (surface terri...

Effets de l’intensité du brûlage dirigé et de la scarification sur la diversité des espèces végétales dans un peuplement de pin gris (Pinus banksiana Lamb)

Vegetation development was studied in a jack pine (Pinus banksiana) clear-cut one and two years after prescribed burning or scarification. Even if differences were found in species composition, mos...

Development of lumber volume recovery correction models for stem deformations of natural black spruce trees

Abstract Simulation methods exist to predict lumber volume recovery based on tree size. In most cases, however, they do not account for stem deformations, leading to possible overestimations of product recovery. This study calculated differences in lumber volume recovery between a stud sawing simulator and a real stud sawmill for natural black spruce trees. Virtual stems were reconstructed from diameter at breast height (dbh) and tree height data. They were processed with Optitek simulator and results were used to build equations predicting product recovery based on the tree size. Predictions from these equations as well as direct results from the Optitek were used to develop correction models predicting the difference between simulated values and observed values from the real sawmill. Four methods were developed to correct estimated lumber volume recoveries. The results indicated that the lumber volume correction models for stem deformations could adjust the predictions of lumber volume recovery from the simulation to be more accurate estimates, especially for large dbh trees (20 cm dbh class or above). With the correction models developed, it was evident that the lumber volume recovery in natural black spruce trees could be estimated directly and accurately using easily measured tree dbh and height from the forest resource inventory.

Morphological indicators of response to overstory removal for boreal conifer trees

Regeneration of forest stands through the preservation of existing advance growth has recently gained considerable interest in various regions of North America. The effectiveness of this approach relies however on the capacity of regeneration to respond positively to overstory removal. Responses of advance growth to release is dependent on tree characteristics and site conditions interacting with the degree of physiological shock caused by the sudden change in environmental conditions. This paper presents a review of the litterature describing the relationships between morphological indicators and the advance growth response to canopy removal. No clear relationship between age, height at release and response to release could be demonstrated. Prerelease height growth has been found to be a good indicator of post-release response for many species. Live crown ratio appears to be a good indicator of vigour for tolerant species. The value of height/diameter ratio would seem of little value in predicting response to release since it varies with site and it is difficult to draw general trends for this ratio based on species tolerance (this contradict some of the text! We need to be more clear here). The ratio of leader length to length of the longest lateral at the last whorl could also serve to describe the degree of suppression before harvest for shade tolerant species. Number of nodal and internodal branches or buds has been found to be related with vigour for many species. Logging damage has been shown to be an important determinant of seedling response to overstory removal. This paper also suggests the use of combined indicators and suggests critical threshold values for these indicators.

Modeling Windthrow at Stand and Landscape Scales

Windthrow results when the wind loads acting on individual trees exceed their stem or anchorage resistance, leading to stem breakage or uprooting. While recurrent windthrow is an important natural disturbance process, it also affects management planning, and can pose a threat to human safety and infrastructure. Empirical approaches to windthrow susceptibility modeling can represent the full range of site, stand, and management conditions across windy landscapes, but provide limited insights into associated mechanisms and are of unknown accuracy in locations or scenarios not represented in the underlying observational data sets. Hybrid empirical–mechanistic approaches provide useful frameworks for integrating knowledge of component processes, but are simplified representations of the biology and mechanics of windthrow and of the range of site and stand conditions within which windthrow occurs. Both empirical and hybrid-mechanistic models have been integrated with forest inventories and growth and yield models within geographic information system (GIS)-based decision support systems that enable researchers to evaluate contrasting management and climate scenarios. However, these models are not yet available in formats that enable easy use by practitioners. Hybrid-mechanistic models could be improved by better representation of the interaction between wind and canopies and among trees, and the process of damage propagation during storms. As well, the models need to be validated for sites that better reflect the range of stand conditions and management regimes in forested landscapes. Wide availability of high-resolution satellite images for change detection, high-resolution spatial weather data sets, and LiDAR for characterizing terrain, soil drainage, and stand structure set the stage for rapid improvement in landscape-scale modeling of windthrow with both empirical and hybrid-mechanistic approaches.